This invention relates to devices, particularly optical devices, for controlling propagation of energy, particularly optical beams, using electric field control. In particular, the invention relates to devices with poled structures, including periodically poled structures, and electrodes which permit controlled propagation of optical energy in the presence of controlled electric fields applied between electrodes. The invention relates to a fundamentally new class of flat panel optical displays.
The current technology for an EO switchable grating is shown in FIG. 1 (Prior Art). In this structure, periodically patterned electrodes serve as the elements that define the grating. The underlying material does not have a patterned poled structure, as hereinafter explained. An input beam 12 is coupled into a electro-optically active material 2 which contains an electrically controllable permanent grating 6. When the voltage source 10 to the grating electrodes is off, the input beam continues to propagate through the material to form the output beam 16. When the grating-controlling voltage source is switched on, an index modulation grating is produced in the material, and a portion of the input beam is coupled into a reflected output beam 14. The material has an electro-optically active poled region 4 with a single domain, with the same polarity throughout the poled structure. A first electrode 6 is interdigitated with a second electrode 7 on a common surface 18 of the substrate. When a voltage is applied between the electrodes, the vertical component of electric field along the path of the beam 12 alternately has opposite sign, creating alternate positive and negative index changes to form a grating. The strength of the grating is controlled by the voltage source connected between the two electrodes by two conductors 8.
A second general problem with the existing art of EO and piezoelectric devices using uniform substrates and patterned electrodes is that the pattern of the excited electric field decays rapidly with distance away from the electrodes. The pattern is essentially washed out at a distance from the electrodes equal to the pattern feature size. This problem is aggravated in the case of a grating because of the very small feature size. Prior art gratings formed by interdigitated electrodes produce a modulated effect only in a shallow surface layer. EO structures interact weakly with waveguides whose dimension is larger than the feature size. While longer grating periods may be used in higher order interaction devices, the lack of sharp definition described above again seriously limits efficiency. The minimum grating period for efficient interaction with current technology is about 10 microns. What is needed is a way to maintain the efficiency of EO devices based on small structures, despite a high aspect ratio (i.e. the ratio of the width of the optical beam to the feature size). Switchable patterned structures are needed which persist throughout the width of waveguides and even large unguided beams.
There are several related technologies in the prior art that use light sources coupled with waveguide structures for display applications.
J. Viitanem and J. Lekkala ("Fiber optic liquid crystal displays," SPIE Vol. 1976, High-Definition Video, pg. 293-302 (1993), and references therein) review the characteristics of flat panel displays that use the waveguide principle coupled with liquid crystal switching. A number of designs are discussed. All have the following common design principles. A modulated light source is mechanically scanned across a series of electro-optically active waveguides that form the row elements of the display. A series of parallel electrodes form the column locations for the display. Light is coupled out of the waveguides and scattered toward the viewer at a column spatial location using the electro-optic effect. Thus a two-dimensional array of pixels is formed. PA1 Another embodiment using the "waveguide tap" method is described in U.S. Pat. No. 5,106,181, April, 1992, and U.S. Pat. No. 5,009,483, April, 1991, Rockwell, III, "Optical Waveguide Display System". Rockwell III discloses a display that uses waveguides to guide light in simultaneous rows. Light is coupled out of a waveguide into the cladding using the electro-optic effect in the cladding. Although the structure is different from that of Viitanem et al. above, this display suffers the same problems of low pixel density and inefficiency discussed above. PA1 U.S. Pat. No. 5,045,847, September, 1991, Tarui et. al. , "Flat Display Panel", discloses another version of the "waveguide tap" method. A planar waveguide structure is used with a layered core material consisting of interspersed layers of a-SiN and a-Si. Light from a laser diode source is confined within the planar waveguide until a voltage is placed across the core of the waveguide. The voltage causes the index of the core to be reduced thus allowing light to escape the waveguide structure. This display suffers from all of the difficulties discussed above. This design has an additional efficiency penalty since all pixels are simultaneously illuminated, but only one pixel at a time is activated. Thus a very small fraction of the light is coupled out toward the viewer at any one time. PA1 U.S. Pat. No. 5,083,120, January, 1992, Nelson, "Flat Panel Display Utilizing Leaky Lightguides" discloses a leaky lightguide used as a row-backlight for a display. Light from a source such as a laser diode is coupled into the lightguide to provide uniform illumination of a row in a display. This backlight is combined with an array of ferroelectric liquid crystal shutters to make the display pixels. In this case the waveguides are only used to replace the fluorescent light normally used in LCD displays, and there are no active "waveguide taps" or switches. PA1 U.S. Pat. No. 4,640,592, February, 1987, Nishimura et al., "Optical Display Utilizing Thermally Formed Bubble in a Liquid Core Waveguide", discloses a display that uses multiple liquid filled fibers or waveguides as rows. These waveguides are placed over a series of heater electrodes. Bubbles are formed in the waveguide core when the heater voltage is turned on, thus scattering light out toward the viewer. The display image is formed when modulated laser light is mechanically scanned sequentially into each row waveguide. Although this approach solves some of the problems inherent in the "waveguide tap" approach by putting small scattering centers in core region, the time scale of thermal processes will preclude this display from having a fast enough frame rate to display full motion video. There are no waveguide switches in this design. PA1 scanning the diverting devices in an upstream sequence if the finite turn-off time of each of the diverting devices reaching the off state from the on state is greater than the finite turn-on time of each of the diverting devices reaching the on state from the off state, and in a downstream sequence if the finite turn-off time of each of the diverting devices reaching the off state from the on state is less than the finite turn-on time of each of the diverting devices reaching the on state from the off state.
In this prior art, light is confined in a waveguide which is composed of a core optical material that has an index of refraction that is larger than the surrounding cladding material. The light, normally confined primarily to the core, is forced to "leak" out of the core of the waveguide at a desired spatial location. The waveguiding effect is destroyed by electro-optically reducing the index difference between the core and the cladding along a certain distance. The electro-optically active material may, in principle, either reside in the core (to reduce the index) or in the cladding (to increase the index). In Viitanem et al., the cladding is active. The technique of destroying the waveguiding effect is called a "waveguide tap" in some of the prior art literature.
The "leaked" light propagates by free space diffraction to a scattering center where it is directed toward the viewer to form a pixel of the display. The light that "leaks" out of the destroyed waveguide is no longer spatially confined but expands in area according to standard diffraction theory as it propagates away from the destroyed waveguide segment. This two-dimensional expansion of the light causes three problems.
First, since the diffraction angle of the light previously confined to the waveguide is relatively small, a long interaction lengths results. (A significant fraction of the optical energy must leave the core region before it can be scattered toward the viewer.) This typically will limit the spacing of the scattering centers to be larger than 1 mm. This effect causes a low resolution display with a low pixel packing density.
Second, the two dimensional expansion of the beam makes it virtually impossible to collect a large fraction of the light on a scattering center and direct it toward the viewer. This causes the display to have a low electrical power efficiency.
Third, the two-dimensional expansion of the beam causes the scattering centers to be large, and hence the pixel size is large. This also degrades the display resolution.
A consequence of the large pixel spacing is that long waveguide lengths must be used to cover enough pixels for a display. The display must then operate in a region where the effects of waveguide loss are large, again reducing efficiency. Thus this prior art design suffers from low pixel packing density, a large pixel size, and a low electrical power efficiency.
What is needed to resolve these problems is the development of a short, efficient, low-loss electro-optic waveguide switch that routes the entire light beam out of the row waveguide and into a narrow solid-angle so that the switched light can be efficiently directed either towards a pixel scattering center or into another waveguide that leads to a scattering center. This will concentrate the light on the scattering center, maximizing both the efficiency of the display and the pixel packing density.
All of the prior art uses parallel input waveguides excited directly by a light source because the switching mechanisms known to the art do not permit the efficient switching light out of a supply waveguide into the desired row waveguide. This accounts for the unwieldy mechanical beam scanning apparatus of Viitanem et al. and Nishimura et al., the simultaneous waveguide bundle illumination of Rockwell III, the planar waveguide excitation of Tarui et al., and the simultaneous diode array illumination of Nelson and Nishimura et al. Yet the simplest architecture for a display, where light is coupled into a single waveguide and routed to the pixels, requires at least two consecutive switches to illuminate a full two dimensional array of pixels: one to select the row waveguide and the other to select the pixel. The prior art can (with difficulty) make the pixel switches. However, the row switch, which must connect waveguide to waveguide, is not possible with the waveguide taps described above. Waveguide to waveguide coupling would just be too inefficient because of the divergence of the light by two dimensional diffraction following the destruction of the waveguide in the switched region. What is needed is a switch which concentrates the switched light efficiently into another waveguide.
The integrated optical channel waveguide communication bus of Becker and Chang, can not be used as a display since it does not include "waveguide taps" or any other method to allow the light propagating in the waveguides to be directed toward a viewer in a pixel format. Furthermore, the switches are fabricated using an interdigitated electrodes structure previously described above in reference to FIG. 1, and they do not contain any pattern poled structures. The switch design has a high switch insertion loss, and hence can not be used in any applications which require a large number of switches on the same waveguide such as a display or a multi-switch data bus for communication applications.
The problems with the prior art are summarized as follows: 1) compact high resolution displays can not be fabricated due to the large pixel spacing, 2) the inefficient "waveguide taps" limit the brightness of the display, 3) the power efficiency is low because of the optical losses, and 4) power cannot be efficiently switched into another waveguide. Since displays are to be used to explain this invention, conventional methods of scanning that are generally applied to such devices are discussed briefly here in order to aid understanding of the invention. Conventional types of displays include cathode ray tubes (CRTs), liquid crystal displays (LCDs), active-matrix LCDs, (AMLCDs), electroluminiscent (EL) and plasma displays. In most of these devices, electrical signals are applied to the matrix of electrodes to control the medium in question. In a CRT, an electron beam is conventionally scanned both horizontally and vertically onto a screen to form a number of display elements. These display elements collectively form an image. Conventional optical scanning systems utilize multifaceted mirrors for deflecting the light beam over a scanning area, which is typically scanned from left to right, on a row-by-row basis.
A conventional twisted-nematic field LCD is made of a cell that contains a liquid crystal material sandwiched between two glass plates. Transparent conductor electrodes are configured on the inner surfaces of the cell to form the dot matrix pattern. A transparent polarizing film is attached to the top glass plate with an adhesive. A second polarizer is bonded to the bottom plate and it is oriented at 90.degree. to the polarization plane of the first mentioned polarizer. The molecular rotation of the liquid crystal material either blocks or allows transmission of light through the liquid crystal material, thereby altering its optical properties. In an OFF state with no voltage applied, polarized light entering the front of the cell follows the direction of the twist and undergoes a 90.degree. rotation as it exits the cell. This rotation allows the polarized light to pass through the rear polarizer unchanged. When a voltage is applied to the cell, the cell is effectively in an ON state. The liquid crystal molecules become oriented parallel to the electric field because the energy of the electric field destroys the twisted structure. The polarized light entering the cell is therefore not rotated and is in fact absorbed by the rear polarizer. Thus the ON state is black, and the OFF state is clear. An LCD display consists of rows and columns of electrodes that are connected to voltage supplies through a driver circuitry. In operation, the display is scanned or multiplexed row by row from top to bottom. The driver circuitry sequentially selects the row lines by giving them a non-zero voltage, while the column lines simultaneously send a non-zero voltage to all the display elements in the selected row. When a proper voltage difference is generated across a row and a column, the liquid crystal material at the intersection of these electrodes, which is known as the selected display element, is activated in the manner described above. The switching time for these types of structures is in the tens to hundreds of milliseconds range.
Conventional active-matrix LCDs provide a switching element at each display element or subdisplay element. For reflective displays, the switching of each display element has been done by using a silicon wafer as the substrate and providing a FET to drive the display element. During the times that a display element is not addressed, the display element has a high impedance and is therefore isolated from the drive circuitry. The display element acts effectively as a capacitor, storing an electrical charge and maintaining a voltage throughout the period during which it is not addressed.
Conventional plasma displays are devices in which the switching element is a positive charged column of a glow discharge in a gas. Each row of the display has a groove in the bottom glass sheet to contain the ionized gas. Plasma addressing relies on the principle that when the gas in a plasma discharge tube is ionized, it acts as a good conductor; and when it is deionized, it is an insulator. When the cathode is activated, the plasma gas ionizes, thereby facilitating an electrical connection between the bottom electrode and ground. The display elements can be written by modulating the data line voltages. When a display element is not being addressed, the respective bottom electrode is electrically isolated such that the display element is not activated. In operation, scan pulses are applied to the column electrodes according to the image stored in memory. The scan pulses are applied to the row electrodes by scanning the rows one at a time sequentially. Once ionization occurs, a current will continue to pass through the plasma even when the voltage is decreased below the threshold voltage. If the voltage is further decreased, light emission will cease.
In conventional electroluminiscent displays, light is produced by applying an electric field to a phosphor. The high electric field inside the material excites carriers, which eventually decay with the emission of light. In thin film EL displays, most of the energy that produces light is delivered during the rise and fall of the driving. A typical mode of operating such a display is to drive columns with positive-going pulses, scan the whole display, and then drive the entire panel with a negative-going pulse, to remove the charge stored in the display elements. Since one display element is addressed at a time, a panel of practical size, for example, 10,000 display elements refreshed at a rate of 50 frames per second, will need to be addressed with a time of at most 0.5 microseconds per display element. Noninstantaneous photoresistor response times as well as RC time constraints arising from the finite capacitance of the electroluminiscent panel make such a fast addressing time difficult to achieve.
It is apparent that the technologies described above are similar in that they depend upon a matrix or array of electrodes individually addressed. Electrical signals applied to the matrix of electrodes control a medium such as a liquid crystal display or plasma display. The image that is seen on the display screen is controlled by applying signals to the electrodes associated each individual display element on the display. U.S. Pat. No. 4,635,082 "Thermo-optic light modulation array" and U.S. Pat. No. 4,281,904 "TIR Electro-optic modulator with individual addressed electrodes" illustrate other such display technologies similar to those described above.
Diverting devices, such as switches, are devices which divert energy which is propagating along a primary path into a secondary path. An ideal switch is one that can be turned on and off instantaneously, without taking any time to reach its full-on state, (with no "rise time"), and without taking any time to reach its full-off state, (with no "decay" or "fall time"). In reality, conventional diverting devices have both a rise time and a fall time, dependent not only upon the nature of the device itself, but also upon the nature of the devices driving it and the criteria governing the full-on and full-off states. The terms full-on state and full-off state will be explained in more detail later.
There are, for example, conventional optical switches incorporating materials with refractive indices which can be altered by the application of an electric or magnetic field, and hence the switching can be achieved by applying a predetermined voltage to the switch. As indicated above, there is a time associated with both the driving unit attaining the required voltage level, a time associated with applying that level to the switch, and a time associated with the material properties and design of the switch itself responding to the applied voltage to cause the diverting of the optical energy to occur. For the same electro-optic switch, removal of the applied voltage does not immediately turn the switch off. There is a time associated with the voltage falling to a level that ultimately turns the switch off, and there is the time once again for the switch to respond to such an instruction by the driving unit to remove the applied voltage.
There are also conventional thermo-optic devices which originate from the temperature dependency of the refractive index. Thermo-optic switching on the order of milliseconds to microseconds becomes possible when light is confined in thin-film waveguides. Such devices fabricated in plastic or polymer materials typically exhibit rise and fall times of less than 0.5 ms. The response of such a device is shown in FIG. 82. The operating waveform or control signal is shown here as a typical bidirectional rectangular pulse, but it will be apparent to a person skilled in the art that other waveforms may be desirable or required to initiate such an operation. As the temperature increases to temperature T1, the refractive index of the material begins to change, when the difference in refractive index is sufficient, the material begins to divert light. As the temperature decreases again, the refractive index of the material returns to its original index, and the light, which is no longer diverted, it will continue along its primary path.
FIG. 82 also illustrates that the switch may not respond instantaneously to the controlling or driving signal, although instantaneous response may be attainable from certain devices. In instances in which a plurality of discrete diverting devices are disposed along an energy propagating in series and accessible on a sequential basis, the concern with rise times and fall times is important, especially when a fast switching operation along the sequence is required. Minimizing the switching time is most important when large-scale arrays of switches are used to route the optical energy must be rapidly redirected between output paths, and or a fast switching operation along the sequence is required signal is modulated at a high rate.
Scanning between a large number of output waveguides inherently requires a rapid redirection of the outputs.
An example of a series of such diversion devices, shown in FIG. 83A, are fed by a common energy source 8300 such that energy propagates from an upstream direction to a downstream direction in the direction of the arrow as illustrated in FIG. 83A. It is apparent that if a control signal for a first diverting device 8302 is asserted to switch the first device 8302 on, it will take a finite amount of time T831 in FIG. 83B for the device itself to actually reach its full-on state, and a finite amount of time T832 for the device to actually reach its full-off state once a control signal is asserted to switch the device off. If a second diverting device 8304 is switched on at time T83 during the time that the first switch is in its full-on state as shown, or in transit from its on-state to its off-state, the second diverting device may have little or no effect. A portion or all of the energy will continue to be diverted by the first switch 8302, before reaching the second switch 8304.
In order to solve this problem, attempts have been made to design switching devices which have extremely short rise and fall times, thus enabling series switching to occur from a common energy source by simply sequentially switching the diversion devices on and off in turn to scan diverting devices in the series. An alternative is the provision of a switching method and apparatus that are able to attain fast scanning of the diverting devices that are disposed along an energy propagating path even if the rise and fall times of the diverting devices are relatively long.
FIG. 85 shows a conventional display 8502 in which the display elements are arranged into a matrix array of rows R1 . . . R6 and columns C1 . . . C6 that may be addressed by using a conventional addressing method. Each display element 8504 occupies a unique location in the matrix which allows individual addressing by a corresponding combination of addressing lines for the rows and columns. Although individual addressing allows each display element to be switched on and off each display element independent of those around it, if the diverting elements are situated along the same optical path from a common source 8506, the switching speed will still be determined by the rise and fall times of each individual switch unless the display elements are each supplied with an individual energy source. Individual addressing requires a substantial number of driving circuits to facilitate such operation. Therefore, there is a need for a method of scanning the display elements with conventional switches without a large number of driving circuits required by individual addressing.
Conventional scanning may be described as the selective addressing of specific display elements, of the RxC matrix or array of display elements performed using column and row addressing circuits. Scanning of the matrix may be performed in many ways, for example, by successively addressing row address lines while simultaneously applying data signals to all of the column addressing lines, or by successively addressing column address lines while simultaneously applying data signals to all of the row addressing lines. This type of scanning does not address the issue of the particular architecture in which each row or column of display elements, for example, is supplied by only one energy source, and sequential switching of the associated column or row is required to divert energy to the individual display elements. Therefore, there is a need for scanning the display elements using conventional switches with finite rise and fall times effectively, with each row or each column of the switches situated along the same optical path from a common energy source.